To investigate the toxicity and potential anti-tumor activity of Thymidine (TdR), a naturally occurring substance, twelve patients with refractory malignant disease were treated with TdR. The toxicities encountered with the dose of 75 g/m2/day TdR given as a continuous infusion for five days were tolerable and reversible. The dose limiting toxicity has been myelosuppression, especially manifested as leukopenia. Thrombocytopenia and anemia were also observed. Other toxic effects were minimal and included gastrointestinal and central nervous system. Although no significant responses were seen in this group of patients with far advanced metastatic disease, minor responses occurred in melanoma and acute leukemia. The number of patients per disease category studied so far is too small for evaluation of antitumor activity. This study is closed.